In many applications, video data from a video source, such as a camera, is sent to a video processing device, such as a digital television, in a serial manner. Often, the video source employs a parallel-to-serial converter (also referred to as a “serializer”) that receives parallel video data and converts it to serial video data. The video source then sends the serial video data to the video processing device via a serial link, which may be configured as a low voltage differential signal (LVDS) link. The video processing device, in turn, employs a serial-to-parallel converter (also referred to as a “deserializer”) to convert the received serial video data into parallel video data. The video processing device may then process the parallel video data to generate an image on a display.
In addition to video data, other types of serial data may be communicated between the video source and the video processing device. For example, the video source may send control data to the video processing device for controlling one or more aspects of the processing of the video data. Additionally, the video processing device may send status or informational data to the video source for use by the latter in performing any of a number of functions. Such data may be transmitted asynchronously in compliance with the universal asynchronous receive/transmit (UART).
Because the video processing device receives both video data and UART data, the video processing device needs to differentiate the video data from the UART data. Typically, the video processing device differentiates the two types of data by their respective frequencies or equivalently by the duration of the pulse or the bit time. For example, video data may be sent with a frequency as low as 15 Mbs. Whereas, UART data may be sent with a frequency of 10 Mbs in order for the UART data to be sent within a video blanking interval. A pulse generator is typically used to transmit the UART data.
However, because of variations in the process of manufacturing integrated circuits, the UART data rate achieved by a pulse generator may vary from 10 to 22 Mbs. In such a case, the rate of the video data has to be at minimum 22 Mbs plus a guard band or margin in order for the video processing device to differentiate between the video data from the UART data. Thus, in such a case, low video data rates, such as 15 Mbs may not be possible. Or, conversely, if low video data rates are required, the UART data rates would have to be significantly decreased, which may result in potential loss of UART data because the required data may not be able to be sent within the video blanking interval.
Thus, in order to accommodate high UART data rates and low video data rates, there is a need to reduce variation in the UART pulse duration by using an improved pulse generator that is more process independent.